The present invention relates to the Karl Fischer water determination method. More particularly, the present invention relates to a one-component Karl Fischer reagent comprising a base component comprising imidazole and a substituted imidazole.
The volumetric analysis of water was developed by Karl Fischer and is based on the oxidation of sulfur dioxide by iodine in the presence of water according to the following chemical equation:2H2O+SO2+I2⇄H2SO4+2HI
Typically, the Karl Fischer titration is performed in the presence of an alcohol as a solvent and a base, with the following chemical reaction presumed to occur:H2O+I2+(RNH)SO3R′+2RN⇄(RNH)SO4+2(RNH)Iwherein RN is a base and R′ is alkyl (optionally substituted).
It is assumed that within the reagent an alkyl ester of sulfurous acid is formed from alcohol and sulfur dioxide. The latter is oxidized under consumption of stoichiometric amounts of water by iodine to the corresponding sulfuric acid alkyl ester. In practice, as alcohol components, mainly methanol and glycol monoalkyl ether are used. (See, E. Scholz “Karl Fischer-Titration,” Springer Verlag 1984). The alcohol used not only serves as a solvent, but also participates in the reaction and hence influences the titration behavior of the reagents produced therewith. The end point titration can be recognized by an excess of iodine, which may be indicated visually, photometrically or electro-magnetically, but a potentiometric recognition of the end point or by a dead-stop indication leads to substantially more thorough results.
There are four different basic forms of the titration according to Karl Fischer: (1) volumetric titration using a one component reagent; (2) volumetric titration with a two component reagent; (3) coulometric titration with a diaphragm; and (4) coulometric titration without a diaphragm. The four variants require different reagents.
For a one component titration, a reagent is necessary which keeps all reaction partners of water in one solution. For the latter purpose, sulfur dioxide, iodine, and the base are dissolved in an alcohol. In this solution, sulfur dioxide and the alcohol react with the base after being put together to form alkyl sulfite. The base accepts the released proton. When a water-containing sample is added to the component, the alkyl sulfite reacts with iodine and water to form alkyl sulfate and iodide. In the original reagent made by Karl Fischer, methanol was used as the alcoholic component. Later on, more stable reagents were yielded by using methyl glycol. In all one component reagents the titrimetric substance, i.e., the concentration of active iodine, decreases with time. As solvent for the sample in the titration beaker, most of the times methanol or a mixture of methanol and other solvent is used.
For a two component titration, two reagents are necessary. As titration component, a solution of iodine is methanol is used and a solution of sulfur dioxide and a base in methanol serves as the solvent. In the latter solvent, as in a one component reagent, alkyl sulfite is formed. The solvent component is placed in the titration beaker and subsequently titration is performed using the titration component. In contrast to the one component titration, in two component titration a decrease of the normality of the titration component does not occur, as long as no humidity penetrates into the holding flask.
The reagents used for the coulometric determination of water content according to Karl Fischer are different from those utilized in volumetric analysis. Instead of iodine, soluble iodide is used, from which during titration iodine is formed by anodic oxidation, which reacts in analogy to the above-described reaction scheme. The further constituents of the reagents are the same as in the volumetric reagents: sulfur dioxide, a base and an alcohol. In the case of the coulometric titration with diaphragm, the cathode and the anode space are separated by means of a diaphragm in the cell. Both spaces have to be filled up with reagent separately, with the cathode space usually being provided with a special cathode reagent. In the case of coulometric titration without a diaphragm, a separation of cathode and anode space is not necessary because the particular geometry of the cathode avoids the formation of substances that might be oxidized.
According to Karl Fischer, the base component of a reagent serves the function to neutralize the acid which is produced, and thereby facilitates a quantitative reaction. In former times, pyridine was used as a base in practice. Looking for toxicologically acceptable, non-harmful bases, EP-B-0 127 740 describes as further suitable bases imidazole, thiazole, pyrimidine, triazine or substitution products thereof. If imidazole was used in one component reagents, however, it was observed that upon longer storage times, especially at higher temperatures as occurring in hot countries, undesirable precipitations or crystals of imidazolium sulfate were formed, which led to problems in the flexible tube systems of the apparatus used for Karl Fischer determinations. Specifically, it was observed that because the plastic tubes of the Karl Fischer instruments were not completely gas tight, ambient air could enter the tubes. The ambient air contains a high amount of water vapor in hot, humid countries and the water can react with the iodine and methyl sulfite of the reagent to form methyl sulfate which can form a precipitate with the imidazolium ion. In order to prevent these precipitations, the Ph.D. thesis of Silke Grunke, Chemistry Department of the University of Hannover, 1999, Chapter 7, proposes to use a substituted imidazole, such as 2-methylimidazole in one component reagents instead of imidazole. However, when using this substituted imidazole, a rapid decay of normality is observed, leading to the situation that such one component reagents are not stable upon storage. The reason for this decay in normality appears directly related to the pH of the component solution. Because 2-methylimidazole is more basic than imidazole, the iodine present can form iodide and hypo iodide, which results in a significant decrease in available iodine and hence a reduction in normality.
Consequently, a need exists in the industry for an improved Karl Fischer reagent which is stable upon storage as a one component reagent regarding normality, and which, especially at high temperatures, does not tend to substantially precipitate.